The Zener diode voltage regulator, worked out and running
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A Zener diode regulator is the simplest circuit that holds a voltage steady: a resistor from a supply that is too high, and a Zener diode from the output to ground, wired backwards, so it conducts once the voltage across it reaches its rated value and holds it there.
The circuit above is the one most classes build. A 12 V supply feeds a 220 ohm resistor, and a 1N4733A, a 5.1 V Zener, sits across the output with a 1 k load beside it. Press Run and the simulator solves it: the output is 4.94 V, with 27 mA flowing through the Zener. Take the load off and the output rises to 4.98 V. Open the editor to change the supply, the resistor or the Zener and watch the numbers move.
How a Zener regulator works
A normal diode conducts one way and blocks the other, until the reverse voltage gets high enough to break it down. A Zener diode is built to break down at a set voltage, and to do it safely. Below that voltage almost no current flows. At it, the current can rise a long way while the voltage barely moves. That flat part of its curve is the whole trick.
The series resistor sets the current. Whatever the supply has left after the Zener’s voltage is dropped across the resistor, so the current through it is (Vin minus Vz) divided by R. That current splits: some goes to the load, and the Zener takes the rest. As long as the Zener gets some current, the output stays near Vz.
That is why the Zener goes in backwards, with its band (the cathode) to the positive side. Put it in the other way and it is an ordinary diode, forward biased: in the circuit above the output drops to about 1.0 V.
Working out the resistor
Start with the supply, the Zener voltage and the most current the load will ever take. Here that is 12 V, about 5 V and about 5 mA (5 V across 1 k). The Zener needs a little current of its own to stay in regulation, a few milliamps for a part like this, so plan for the load’s current plus some margin, say 30 mA in total.
R = (Vin minus Vz) / I = (12 minus 5) / 0.03, about 230 ohms. 220 ohms is the nearest common value, which is what the circuit uses. With the load on, the resistor carries about 32 mA: the load takes 5 and the Zener the other 27.
Then check the power. The resistor drops about 7 V at 32 mA, about 0.22 W, so a quarter watt part is at its limit and a half watt part is the safe choice. The Zener’s worst case is with no load at all, when it takes the whole 32 mA at 5 V: about 0.16 W, well inside a 1 W 1N4733A.
The load line, and what the load costs
Draw current against output voltage. The resistor is a straight line: (12 minus Vout) / 220, from 54.5 mA at 0 V down to nothing at 12 V. The Zener is a curve that is almost vertical near 5 V. The circuit settles where they cross.
Adding the load takes current away from the Zener, so the crossing slides down its curve. The curve is not perfectly vertical, so the output falls a little: from 4.98 V to 4.94 V here, about 36 mV for the 5 mA the load took. That slope, about 7 ohms, is the Zener’s dynamic resistance, the number a 1N4733A datasheet calls Zzt. It is why a Zener regulator is good for a reference of a few milliamps and poor for anything that draws a lot.
Try these in the editor
Turn the supply down. At 9 V the output is still 4.84 V, with 14 mA left for the Zener. At 6 V it falls to 4.70 V and the Zener gets barely a milliamp. At 5 V the Zener has given up completely and the output is just the divider of 220 ohms over the 1 k load: 4.10 V.
Turn the supply up. At 15 V the output is 5.04 V, and the Zener takes 40 mA. A higher supply makes the regulator work harder, not better.
Shrink the resistor to 47 ohms. Now the Zener carries about 129 mA at 5.65 V, nearly three quarters of a watt. On a real 1 W part that is hot, and with any less resistance it is the first thing to fail.
Swap the Zener. Pick a 1N4740A, a 10 V part, in the properties panel and the output follows the new voltage until the supply can no longer keep it in breakdown. With 12 V, 220 ohms and the 1 k load, the divider alone gives less than 10 V, so it settles at 9.75 V with the Zener barely conducting.
Zener regulator or a regulator chip?
For a reference voltage, protecting an input, or a few milliamps, a Zener and a resistor are cheap and fine. For powering a board or a module, use a linear regulator such as a 7805 or an LM1117, or a switching regulator. They hold their output within a few millivolts across a wide range of loads and waste far less power than a Zener burning off whatever the load does not use.
A good rule: if the load current changes by more than a few milliamps, or the supply varies a lot, a Zener alone is the wrong tool.
Common mistakes
The Zener the wrong way around. The band goes to the positive side. Backwards, it clamps the output to about 1 V.
No series resistor. Without it nothing limits the current, and the Zener or the supply fails.
A resistor that is too big. Then the load starves the Zener of current and the output sags below Vz, as it does here when the supply drops to 6 V.
Forgetting the power. Check both the resistor and the Zener at the worst case, which for the Zener is no load.
Questions
How do I calculate the resistor for a Zener regulator?
R = (Vin - Vz) / (I load + I Zener). For 12 V in, a 5.1 V Zener, a 5 mA load and about 25 mA for the Zener, that is about 230 ohms, so use 220. Then check the resistor’s power: about 7 V times 32 mA, 0.22 W.
Why does the output change when I add a load?
The load takes current from the Zener, and the Zener’s voltage falls a little with its current. The fall is its dynamic resistance times the current moved: about 7 ohms times 5 mA, roughly 35 mV, in the circuit above.
Which way does a Zener diode go?
Cathode, the end with the band, to the positive side. It works in reverse breakdown. Wired forward it acts like an ordinary diode and drops about 0.7 to 1 V.
Can I simulate a Zener regulator online for free?
Yes. The circuit above runs in your browser with no account. Open it in the editor to change the supply, the resistor, the load and the Zener model.
Keep going
Build this for real
Open the editor, change a value and watch the number move with it. Nothing to install, and no account needed.